In recent years, fine processing technologies including lithography have achieved remarkable progress. In association with this progress, applied studies have been increasingly conducted, of a sub-wavelength diffraction grating having a period structure less than a wavelength to be irradiated. Examples of the known applied studies include a guided-mode resonance filter (also referred to as “guided-mode resonant filter”) utilizing a diffraction grating (e.g., Non-Patent Literatures (hereinafter, abbreviated as “NPLs”) 1 and 2).
NPL 1 discloses a guided-mode resonance filter utilizing a diffraction grating. Further, NPL 2 discloses a guided-mode resonance filter with a plane diffraction grating having two different dielectric constants, in which a condition is known where guided modes are coupled at waveguide or diffraction light by the diffraction grating.
As for the guided-mode resonance filter, several basic structures are known. FIG. 1 is an explanatory sectional view of a guided-mode resonance filter. FIG. 1A is a sectional view of guided-mode resonance filter 10 having substrate 12 and diffraction grating part 14, and FIG. 1B is a sectional view of guided-mode resonance filter 20 having substrate 22 and waveguide layer (diffraction grating) 24 formed on substrate 22. It is noted that hatching is omitted in FIGS. 1A and 1B.
As illustrated in FIG. 1A, guided-mode resonance filter 10 of this type has substrate 12 and diffraction grating part 14 disposed on substrate 12. Diffraction grating part 14 has a plurality of high refractive index parts 16 and a plurality of low refractive index parts 18. In guided-mode resonance filter 10 of this type, diffraction grating part 14 itself functions as a waveguide layer. Guided-mode resonance filter 10 illustrated in FIG. 1A has a simple structure, and exhibits high reflection diffraction efficiency in a peak wavelength for TE polarized light. On the other hand, this guided-mode resonance filter 10 has high Fresnel reflection for TM polarized light, and thus exhibits low transmission diffraction efficiency.
Further, as illustrated in FIG. 1B, guided-mode resonance filter 20 of this type has substrate 22 and waveguide layer 24 disposed on substrate 22. Waveguide layer 24 has a plurality of high refractive index parts 26 and a plurality of low refractive index parts 28. In guided-mode resonance filter 20 illustrated in FIG. 1B, the refractive index of substrate 22 (nsub) is set at 1.52, the refractive index of high refractive index part 26 (nH) is set at 2.20, the refractive index of low refractive index part 28 (nL) is set at 1.80, diffraction grating period (Λ) is set at 359 nm, diffraction grating filling factor f is set at 0.50, and the depth h of low refractive index part 28 is set at 267 nm. The results of the calculation of transmission diffraction efficiency and reflection diffraction efficiency by RCWA method in the case where TE polarized light and TM polarized light enter the diffraction grating are shown in FIGS. 2A and 2B.
FIG. 2A is a graph showing the transmission diffraction efficiency or reflection diffraction efficiency for TE polarized light in the range of 600 to 700 nm of light wavelength λ, and FIG. 2B is a graph showing the transmission diffraction efficiency and reflection diffraction efficiency for TM polarized light in the range of 600 to 700 nm of light wavelength λ. The abscissa in FIGS. 2A and 2B indicates light wavelength λ (nm). In addition, the ordinate in FIG. 2A indicates the transmission diffraction efficiency (%) or reflection diffraction efficiency (%) for TE polarized light, and the ordinate in FIG. 2B indicates the transmission diffraction efficiency (%) or reflection diffraction efficiency (%) for TM polarized light. The solid line in FIG. 2A indicates the transmission diffraction efficiency for TE polarized light, and the broken line indicates the reflection diffraction efficiency for TE polarized light. The solid line in FIG. 2B indicates the transmission diffraction efficiency for TM polarized light, and the broken line indicates the reflection diffraction efficiency for TM polarized light. As shown in FIGS. 2A and 2B, in TE polarized light, the peak wavelength λ of a reflection spectrum was 650 nm, whereas in TM polarized light, the peak wavelength λ of a reflection spectrum was 640 nm
Thus, a guided-mode resonance filter in which a diffraction grating made of media having two different refractive indexes is formed on substrate 22 is generally known to function as a reflection type band pass filter (notch filter) for each of TE polarized light and TM polarized light in wavelengths distant a little. In this case, large refractive index difference is generated in each of diffraction grating-air interface and diffraction grating-substrate interface, leading to large Fresnel reflection, and thus, in a wavelength having the maximum reflection diffraction efficiency for TM polarized light, the lowering of the transmission diffraction efficiency for TE polarized light is considerably large, which therefore causes the filter not to function sufficiently as a polarizer. In order for a polarizer to function sufficiently, it is preferable that, in a predetermined wavelength region, the reflection diffraction efficiency for TE polarized light is close to 100%, and at the same time the transmission diffraction efficiency for TM polarized light is close to 100%.
Techniques of reducing Fresnel reflection generated in each of the diffraction grating-air interface and the diffraction grating-substrate interface as described above are disclosed (refer to, e.g., Patent Literature (hereinafter, abbreviated as “PTL”) 1). PTL 1 discloses a wavelength filter including a substrate having fine irregularities, and a dielectric layer that covers the fine irregularities. In the wavelength filter disclosed in PTL 1, the fine irregularities are sized so as not to cause higher-order diffraction light for the reduction of the Fresnel reflection, and the cross-section of the fine irregularities is shaped to be triangle, to thereby gradually change the average refractive indexes for the dielectric layer and the air.